1. Field of the Invention
The present invention relates to a dielectric ceramic composition and, more particularly, to a ceramic composition useful as a dielectric material for monolithic ceramic capacitors.
2. Description of the Prior Art
As a high permittivity material for ceramic capacitors, there have widely been used those mainly comprising BaTiO.sub.3 and containing an additive such as BaSnO.sub.3, CaZrO.sub.3 and SrTiO.sub.3 incorporated therein because of their high dielectric constant ranging from 2000 to 15000.
However, these dielectric materials have such a disadvantage that they have a high sintering temperature of 1300.degree. to 1400.degree. C. This results in an increase in the cost required for sincering. In addition, when producing monolithic capacitors with such a dielectric material, it is required to use a material for internal electrodes which does not melt, oxidize, or react with the dielectric material at a high temperature of more than 1300.degree. C. since monolithic ceramic capacitor are produced by the steps of preparing ceramic green sheets, forming an internal electrode layer on each ceramic green sheet, bonding a plurality of green sheets by thermocompression, and firing the resultant multilayered green ceramic body. Accordingly, noble metals such as palladium and platinum have widely been used as a material for internal electrodes of the monolithic ceramic capacitors. However, the use of the noble metals has become a bar to reduction of the cost of monolithic ceramic capacitors since the greater part of the cost thereof is occupied by the internal electrodes.
To solve such problems, it has been proposed to incorporate a vitreous material containing oxides of B, Bi and Pb into a BaTiO.sub.3 -based dielectric ceramic composition to lower the sintering temperature. The dielectric ceramic composition containing such a vitreous material makes it possible to use relatively inexpensive Ag-Pd alloys as a material for internal electrodes of monolithic ceramic capacitors since it can be sintered at a low temperature of 1100.degree. to 1150.degree. C. However, the incorporation of the vitreous material causes such a problem that the dielectric constant lowers, resulting in an increase in size of the monolithic ceramic capacitors. Accordingly, the advantages of the use of the inexpensive electrode material is cancelled.
In addition, the monolithic ceramic capacitors comprising the BaTiO.sub.3 -based dielectric ceramic composition have such a problem that the capacitance thereof is affected greatly by a biasing electric field of a direct current. For example, the capacitance is lowered about 30 to 80% of the initial value by applying a high direct current voltage of 5 kV per 1 mm thickness of the dielectric layer.
To solve such problems, many investigations have been made on lead-based complex perovskites mainly comprising lead because of their low temperature sintering. Some lead-based complex perovskites have been already known to be dielectric ceramic compositions having a relative dielectric constant of more than 10000 and a sintering temperature of 1050 .degree. C. or below. However, a problem of these compositions is that they cause a large temperature coefficient of capacitance.